Synchronous machines, i.e. synchronous motors and synchronous generators, typically comprise a rotor and a stator. The rotor and stator interact electromagnetically such that rotation of the rotor is obtained in case the synchronous machine is a motor, and such that current is induced in the stator coils in case the synchronous machine is a generator.
A number of fault conditions may occur in a synchronous machine, both in the stator and in the rotor. Such fault conditions may instantaneously introduce noticeable deviations in behaviour of the synchronous machine, or they may go unnoticed for a longer period of time, which only after months or even years manifests itself in a catastrophic breakdown.
Traditionally, stator current and synchronous machine vibration measurements have been preferred monitoring methods for determining the presence of a fault condition. Stator current measurements must however be performed under the high voltage conditions of the stator, which therefore requires expensive sensor equipment able to withstand such conditions, in addition to safety considerations for undertaking such measurements. Furthermore, it has been shown that vibration levels do not reflect synchronous machine conditions in a satisfactory manner.
CN102636751 A discloses a method where current that magnetises the electromagnets of the rotor is measured. This current is provided by an exciter, which is a device that feeds current to the rotor windings to magnetise the electromagnets of the rotor measured at the main machine rotor. The current is measured at the rotor coils to determine stator winding turn-to-turn short circuits and rotor winding turn-to-turn short circuits. The method utilises Fast Fourier Transform to extract the harmonics of the field current.
The measurements that are required in CN102636751 A may however be difficult to implement, and the utilised sensors must be able to withstand relatively high currents.
The paper “The detection of stator and rotor winding short circuits in synchronous generators by analysing excitation current harmonics”, by Penman J et al: Opportunities and advances in international electric power generation, International conference on (conf. publ. no. 419) Durham, UK 18-20 Mar. 1996, discloses that the harmonic components present in the generator excitation current may be used to detect stator and rotor winding short circuits. It is disclosed that the machines under test have a rotating excitation system, and it is reasoned that it should be possible to detect the presence of rotor short circuits by inspection of the harmonic components in the exciter stator. FIG. 8 of this disclosure shows the exciter stator current harmonic components plotted for different frequencies both under normal conditions and in the event of stator inter-turn faults, while it is concluded that from this graph that it can be seen that it may indeed be possible to detect rotor short circuits through examination of exciter current harmonics.